The current invention relates to an angle encoder and a method for angle determination.
It is known to use detectors or transmitters for detecting state changes, e.g. for angle determination of a rotatably supported component. In this connection, the use of slide- or layer potentiometers is widely known. With a slide potentiometer, for example, the relationship between the length of a wire resistor or layer resister and its resistance value is utilized in this connection. The wear and tear caused by the slide contact and the maintenance cost connected with this turn out to be disadvantageous in this connection. For the sake of freedom from maintenance, it is therefore desirable to use contactless angle encoders.
Due to strained environmental conditions, preferably magnetic sensors are used in the automotive field, in particular Hall sensors, anisotropic magnetoresistive sensors (AMR) or so-called giant magnetoresistive sensors (GMR). Embodiments with AMR sensors are known for angle measurements up to 180xc2x0. For angle measurements up to 360xc2x0, it is normal to use crossed Hall sensors. Finally, embodiments with Hall sensors are known in which a linear relationship between an angle ( less than 180xc2x0) and an output signal can be achieved by means of suitable flux guidance of the magnetic field. For all Hall variants, offset-free and temperature-stable Hall elements are required, which exist in a variety of forms. These elements usually have an adjustable zero point and an adjustable sensitivity.
In all known methods that use Hall elements for angle measurement, tolerances, play, and temperature influences represent serious difficulties in actual use since distance changes between a magnet and a sensor or (for example temperature-induced) sensitivity changes of the sensor cause signal changes which then lead to a measurement error. Thus for example, the crossed Hall elements are in fact suitable for a 360xc2x0 measurement, but are simultaneously very sensitive to position changes of the axle which is why very precise bearings are required in this connection. This result in higher costs. With conventional angle encoders, methods are known (for example resolver circuit, Philips AMR evaluation circuit, sigma-delta method) in which the temperature-dependent amplitude is eliminated. Tolerances or play in the axial bearing, however, cannot be compensated for by means of this method.
Previously known embodiments which are relatively insensitive to axial offsets have the disadvantage that they can only be used for angle measurements up to 180xc2x0 and furthermore, are temperature-dependent in field sensitivity. With regard to AMR sensors, it must be noted that these are likewise very temperature-dependent and therefore when sensors of this kind are used, expensive compensation processes must be carried out.
DE-P 196 40 695 has disclosed a contactless magnetoresistive sensor which functions with two AMR sensor elements that are rotated in relation to one another by 45xc2x0. To build a magnetoresistive angle sensor with a temperature-stable zero point, the progression of the output signal of the sensor is determined over a presettable angle range with at least two different temperatures of the sensor, whereupon the output signal progressions are set in relation to one another in order to determine a reference angle at which the output signals deviate only slightly from one another as a result of a slight temperature dependency, and a reference angle found in this way is selected as the zero point in subsequent measurements. This relatively expensive method exemplifies the efforts that are usually required in the compensation for tolerances and temperature changes in sensors of this kind.
The object of the current invention is to produce an angle encoder which is not sensitive to axial offsets or tolerances and at the same time permits a measurement of angles of up to 360xc2x0, as well as a corresponding method for angle determination.
With the angle encoder according to the invention, it is possible to easily compensate for axial offsets or component tolerances, wherein it is possible to measure angles of up to 360xc2x0. Manufacture-induced or temperature-induced effects, e.g. tolerances and play, can be compensated for in a simple manner.
According to at preferred embodiment, the magnet of the angle encoder is embodied as cylindrical, wherein the flux-conducting parts encompassing the magnet are embodied as essentially quadrant-shaped. Since the flux-conducting parts and therefore the Hall elements disposed between them can rotate in relation to the magnet, this assures that the distances between the individual flux-conducting parts or Hall elements and the magnet do not change during a rotation. This permits a particularly simple evaluation of the measurement signals.
The Hall elements are suitably disposed respectively offset from one another by 90xc2x0. As a result, in respectively opposed Hall elements, corresponding sine or cosine signals are obtained, which can easily be evaluated in order to obtain an angle value that is independent of tolerances and temperature.
The magnet suitably has a diametrical magnetization. As a result of this, the magnetic field induced in respectively opposed Hall elements has essentially the same magnitude.
In order to further attain the object according to the invention, a method is proposed for angle determination of an angle between a sensor device and a magnetic field through the use of an angle encoder according to the invention. Deviations of the signals picked up by the Hall elements from the pure sine or cosine form, which result from tolerances or plays in the angle encoder, can be easily compensate for by means of the method proposed. Errors that occur can be almost completely corrected by means of the method proposed.
According to a preferred embodiment of the method, a sum signal is respectively formed from at least two Hall voltages proportional to the sine of the angle to be determined and from at least two Hall voltages proportional to the cosine of the angle to be determined, and the sum signals thus obtained are supplied to a sine-cosine evaluation circuit for arithmetical determination of the angle to be determined. This method considerably reduces the occurrence of angular errors in comparison to conventional methods. In relation to a single evaluation, i.e. the determination of the angle by means of only one sine signal and one cosine signal, a reduction of the angular error by a factor of 20 was attained with the use of the angle encoder according to the invention.
According to another preferred method, for at least two pairs of Hall elements, the one of which has a Hall voltage proportional to the sine of the angle to be determined and the other of which has a Hall voltage proportional to the cosine of the angle to be determined, the angle to be determined is arithmetically determined by means of a sine-cosine evaluation circuit. As a result, the angle encoder according to the invention can easily be used as a redundant 360xc2x0 sensor.
In a suitable fashion, the average is calculated from the at least two values determined for the angle to be determined. This in turn permits a very precise determination of the angle to be determined.
In a suitable fashion, the arithmetical determination of the angle to be determined is carried out based on the signals that are respectively proportional to the sine or cosine of the angle, by means of a determination of the associated arc tangent. A calculation of this kind by means of the arc tangent is suited for an arithmetical determination of the angle to be determined based on the above-mentioned sum signals as well as for the arithmetical determination based on a respective sine value and cosine value.
Preferred embodiments of the invention will now be explained in detail in conjunction with the accompanying drawings.